Any idea how a 15mm offset 20x10 would work, offset is the depth they in the wheel well correct?
First off...when there is no + or - sign in front of the offset number, it is taken as a + or positive.
15 mm offset would, in effect, move the wheel (assuming your existing offset is 25.5mm) 10.5mm outboard from your car. Now 10.5mm is only .4" (a bit more than 3/8") but with a wider tire you would in effect be moving the outside face of the tire the 3/8" plus half the increase in width between your existing tire and your new tire.
For example, if you currently have 245's (approx. 9.65"wide) and you put on 275's(approx. 10.83") you would be gaining 1.18" in total width. Half that width(since half is inward and half is outwould from the centerline of the tire) is .59 or about 9/16"+.
So in effect you would be ending up with 9/16" + 3/8"(due to offset difference) or roughly 7/8" total that he outside surface of the tire would be moving towards you fender. That may very well still be within your fender in the front. But that is how it works.
So, not that it is, but if your front tire was currently 1/2" inside your fender, it would end up sticking out beyond you fender.
Bottom line...when changing wheel offsets and tire/wheel widths, BOTH the inner clearance as well as the location of the tires outer surface must be kept in mind.